The present invention relates generally to methods for dehydrating natural gas streams in order to control the formation of hydrates in downstream gas lines. More particularly, the present invention is concerned with a method for dehydrating natural gas with low temperature refrigeration by isentropic expansion of the gas stream.
Dehydration is the process for removing water vapor from the gas stream. Water vapor is an undesirable impurity in natural gas for several reasons. When cooled below its dewpoint, water vapor condenses to liquid water which will itself accelerate corrosion and reduce gas transmission efficiency through elements in the natural gas pipelines and plant equipment. Also, quantities of H.sub.2 S and CO.sub.2 may be entrained in water particles and, thus, cause further corrosion problems. Finally, liquid water and water vapor can combine with low molecular weight hydrocarbons to form solid, crystalline hydrates which plug valves, fittings, and gas lines. Dehydration devices are typically employed at the well head, and on offshore platforms it is especially important that equipment be explosion proof and as compact as possible.
Problems caused by entrained water in gas lines are well known in the prior art, and several control techniques have been suggested. These methods include refrigeration, absorption, and centrifugal separating or "scrubbing". Unfortunately, prior art devices utilizing these methods are typically very large, expensive, and require multiple moving parts and auxiliarly energy sources.
Refrigeration dehydration devices typically employ isothermal processes with natural gas streams and, thus, require large coolant reservoirs or bulky heat exchangers and electrically powered compressors. These elements take up much space in the limited confines of an offshore rig and increase the risk of explosion from stray electrical voltages and failure of moving compressor elements. Further, prior refrigeration methods often cause water to freeze out in solid form. Electrical or gas heating means are often then required to melt the water to liquid form for removal.
Absorption dehydration, while presently one of the most commercially favored methods, requires extremely complicated processing systems. Water vapor is removed from the natural gas stream by absorption into a hydroscopic liquid, typically triethylene glycol (TEG). Water vapor and some hydrocarbons are so absorbed as they flow through a series of bubble cap trays in a contactor means. Rich, or water ladden, TEG then passes through several filtering and regeneration stages to remove the water and entrained hydrocarbons. Often, several pumping stages are also needed before the lean TEG is returned to the contactor means. Thus, this method also requires a large amount of space. Further, the risk of explosion is increased since not only are there many moving mechanical elements subject to failure, the TEG regeneration stages may require electrical or gas heating means.
Centrifugal scrubbing separates water from natural gas by taking advantage of the difference in their densities. While this may be accomplished by relatively compact machinery, as compared to absorption devices, it often requires extremely high speed, electrically powered rotors to centrifugally separate out the water. Again, stray electrical voltages and rotor failure present a significant risk of explosion.
Thus, the need has arisen for a compact and relatively explosion-proof means of dehydrating well head natural gas. It is of course also desirable that such a dehydration means be inexpensive, mechanically simple, and have a low operating cost. Further, since the natural gas producing industry as a whole has shown inertia and reluctance to implement new production techniques, any such improved dehydration means must have proven or readily apparent reliability.
While it is known to use a turboexpander means in conjuction with the processing of hydrocarbons and natural gas, the purpose and effect of the subject invention are not taught. Turboexpanders have been used where light hydrocarbons are recycled and stockpiled as refrigerants in systems concerned with separating out various liquid hydrocarbons of different temperatures of vaporization. Extremely low temperatures are required, and the light hydrocarbons change phase several times as they are intermixed and separated from diverse fluids.